David Winer on Portable Social Networks
"There are enormous economic incentives for companies that run social networks to not let users of other networks access their services. Shareholder value is a function of how many users they have, how they are "monetized" and how hard it is to switch. The harder it is to switch, the more money each user is worth. Any exec that did anything to decrease the number of users they control would probably be fired. So anything that depends on this isn't very likely to happen, in existing networks.
However, a network that, from Day One, allows users of other networks to participate, and allows developers to access user's data, with the user's permission, but without permission from the network, may become the www of open identity systems. As much as it is considered politically incorrect in the tech world to say this, don't bet on Open ID being that network. You would have gotten roasted in 1991 for saying OpenDoc wasn't the future, but it wasn't. For the same reasons OpenID isn't." (http://www.scripting.com/2007/08.html)